Local Resource Competition Research Articles

Evolution of Colony Characteristics in Social Insects. I. Sex Allocation

The inclusive-fitness model gives a powerful method for analyzing the sex-allocation patterns of social insects, both at the level of individual colonies and at the level of populations. Genetic relatednesses and reproductive values, both of which are essential in constructing the inclusive fitness, are relatively easy to estimate empirically. The reproductive success of males and females, however, can in many cases be difficult to estimate. This is particularly so when workers should be counted as a sexual investment, because colonies either recruit daughter queens as new reproductives or form new colonies by fission. My results suggest that we can predict equilibrium investment ratios in such cases, provided that workers are counted as an investment. When daughter colonies are formed by budding and they remain in close contact with the mother colony and with each other, the fitness consequences can be complicated, and the expected investment ratios depend on local resource competition or local resource enhancement. I suggest various alternative approaches for empirically testing the queen-worker conflict over sex allocation. The most promising approaches are those focusing on single species and comparing sex ratios produced by different colonies and populations. The inclusive- fitness model elaborated in this article can be applied to derive the expected allocation patterns, both as a function of the type of colony (number of reproductives in it) and as a function of the amount of resources the colony can use in sexual production.

Influence of social factors on sex ratio at birth, maternal investment and young survival in a prosimian primate

In order to determine whether social factors influence sex ratio at birth in lesser mouse lemurs, experiments were conducted during 5 successive breeding periods on 51 females. At the beginning of the breeding season, females were either isolated (I) or grouped (G) in heterosexual groups with an increasing number of females (2, 3 or 4). To ensure mating, I females were introduced in a group only during the oestrous period. After mating, both I and G females were isolated during pregnancy and lactation. Reproductive capacities of females in terms of oestrus occurrences (n = 324), impregnations (n−210), pregnancies (n = 136) or abortions (n = 38) or litter sizes (1–3 young) were affected neither by age and parity of females nor by group housing prior to conception. G females produced significantly more sons than daughters (67% males for 189 newborn) while females living alone except during the mating period demonstrated a significant inverse tendency (39.6% males for 96 newborn). Distribution of sexes in litters was statistically different from random and varied according to the shift of sex ratio at birth. In G females, the shift in the sex ratio towards males was consistent across the different groups, independent of the number of females living together, suggesting that the presence of only 1 female is sufficient to induce a bias in the sex ratio. No correlation was found between infant survival at weaning and age, parity or group housing of the mother. The maternal investment allocated to male or female newborn was similar provided the litter contained at least 1 male. In litters without males, growth and survival of female infants were significantly less. These results on sex ratio bias in captive female mouse lemurs agree with directions of bias predicted by the local resource competition model for facultative sex ratio adjustment (Clark 1978). Nevertheless, the pattern observed in mouse lemurs appears to be independent of the nutritional state of the female and of differential maternal investment.

Reproductive Investment and Allocation Ratios for the Ant Leptothorax longispinosus: Sorting Out the Variation

Sex-ratio theory as applied to social Hymenoptera has implicated a large number of factors that can affect predictions about sexual allocation. I examine these factors for the tiny forest ant Leptothorax longispinosus by applying Sewall Wright's path analysis to 10 yr of data from two geographical locations. Populations in Vermont and New York were similar with respect to nest sizes and the patterns of queen number but differed strongly in reproduction parameters. Nests in Vermont reared smaller sexuals and produced broods that were more malebiased than their counterparts in New York. Path analysis allowed the exclusion of differential rearing costs, local resource competition, and local mate competition; furthermore, it implicated conflict between queens and workers over alate masses, total investment in sexuals, and allocation to females. Despite considerable explanatory power, variation residual after path analysis showed that unmeasured parameters were important for reproduction patterns in this speci...

Fragile Sons or Harassed Daughters?

In most mammals, juvenile males tend to be more vulnerable to starvation than females and consequently experience a higher mortality. This has been attributed to selection on high male growth rates in response to strong intrasexual competition. Among primates, by contrast, it has been noted that females tend to be less viable as juveniles. This has been attributed to a greater ability of juvenile males to contest for food. An alternative explanation is that there is local resource competition, and adults of the resident sex (usually females) attempt to limit the recruitment of unrelated immatures of the same sex by harassing them. These ideas are not mutually exclusive. A set of predictions from these three hypotheses was derived for two social systems and two levels of food supply. They were tested using estimates of juvenile mortality and juvenile sex ratios of non-human primates based on over 40 data sets drawn from the literature. The results indicate that the local resource competition hypothesis provides the best explanation for the observed patterns in differential juvenile mortality in primates. The contrast between the findings for primates and those for other taxa is attributed to the low growth rate of immatures and the widespread occurrence of conditions conducive to local resource competition in primates.

Sex Ratio in Primates: A Test of the Local Resource Competition Hypothesis

Sex Ratio in Primates: A Test of the Local Resource Competition Hypothesis

Sex-Ratio Variation in Marsupials

Many marsupials produce sex ratios biased towards male or female young. In several cases these changes are comfortably accommodated in the existing theory of sex allocation. Local resource competition and the Trivers-Willard hypothesis have been usefully applied to several data sets, and preliminary experimental work has supported the main tenets of theory. By contrast, several data sets lack explanation, and provide challenges to theoreticians. The high frequency of bias in marsupials does not result from data-dredging, as bias is usually reported in descriptive accounts of marsupial reproduction, without recourse to any theoretical or mechanistic explanations. It is not possible to distinguish whether the marsupial mode of reproduction is well suited to manipulate sex allocation, or whether it facilitates measurement of biased sex allocation. As for most eutherians and birds, the mechanism of prenatal sex allocation is unknown for any marsupial. However, the current interest in sex-determining mechanisms in marsupials suggests a profitable avenue for collaboration between geneticists, physiologists and evolutionary ecologists.

Social rank and sex ratio of captive long‐tailed macaque females (Macaca fascicularis)

Variation in birth sex ratios in primates can be accounted for by two hypotheses: the local resource competition hypothesis [Silk: American Naturalist 121:56-66, 1983] and the hypothesis of Trivers & Willard [Science 179:90-92, 1973] concerning the maternal effect on the quality of a male. We examined the effects of female dominance rank on aspects of reproduction in three well-established captive groups of long-tailed macaques (Macaca fascicularis). High-ranking females produced a higher proportion of sons than low-ranking females, and factors other than rank did not have significant effects on birth sex ratios. Interbirth intervals following daughters were longer than those following sons, but they were independent of the mother's rank. The sons of high-ranking mothers had better survival prospects than sons of low-ranking mothers in some of the groups; no such difference was found for daughters. Overall, there was no sex difference in survival up to 5 years of age. These results support the Trivers-Willard hypothesis rather than the local resource competition hypothesis. An analysis of interbirth intervals suggested that the deviation in birth sex ratio is already established at conception.

Variation in Sex Ratios of Banner-Tailed Kangaroo Rats in Relation to Population Density

Over a 5-year period, sex ratios of juvenile Dipodomys spectabilis varied significantly at one study site, whereas there was a recurrent and significant male bias at a second site. Population density was variable at the first site, but at the second site it was consistently at high levels. The proportion of males was correlated positively with population density. Demographic and dispersal data revealed no variation in competition for local resources or in differential mortality that ultimately could account for the variation in sex ratio. However, male weights relative to female weights at the end of parental care were lower where sex ratios were male-biased, suggesting a correspondence between male-biased sex ratios and relatively cheaper males. The adjustment to male-biased sex ratios apparently occurred in utero or during lactation. A hypothesis that might explain the variation in sex ratio is that it is an adaptive response either to variation in the relative costs of producing sons and daughters or to variation in the life-history expectations of sons and daughters that occurs with variable population density. Another hypothesis is that competition among siblings results in higher mortality in one sex, and this effect is more pronounced at higher population density.

Dispersal and the sex ratio at birth in primates

The females of many species of primates settle for life within the home ranges of their mothers, whereas males disperse as immatures. According to the theory of sex allocation, the costs incurred by mothers through local competition for resources with their philopatric daughters should favour the evolution of male-biased sex ratios at birth. I report here two tests of this hypothesis based on data from 15 genera of primates. First, I show that the intensity of competition for resources within kin groups is strongly and positively correlated with sex ratios at birth. Second, I show that sex ratios at birth are higher in genera with female-biased philopatry than in genera in which philopatry is not female-biased. These analyses suggest that local resource competition among kin powerfully influences the evolution of sex ratios in primates.

Seed size variation: magnitude, distribution, and ecological correlates

We examined seed-mass variation in 39 species (46 populations) of plants in eastern-central Illinois, USA. The coefficient of variation of seed mass commonly exceeded 20%. Significant variation in mean seed mass occurred among conspecific plants in most species sampled (by hierarchical ANOVA), averaging 38% of total variance. For most species, within-plant variation was the larger component of total variance, averaging 62% of total variance. Variation in seed mass among fruits within crops was significant in most species tested. We conclude that variation in seed mass among and within plants is widespread and common. There was little evidence of trade-offs between number of seeds and mean or variance of seed mass, and little correlational evidence of local competition for maternal resources. No consistent ecological (dispersal mode and growth form) correlates of variance of seed mass were evident.

Male-biased reproduction and sex-ratio adjustment in muskrats.

Sex ratios of a population and of litters were sampled in muskrats in Ontario, Canada. Sex ratios of litters sampled from nests were male biased (54% male). Until weaning, no differential costs of producing and rearing male and female young were identified that could account for this greater production of males. Following weaning, however, male-biased dispersal of juveniles from their natal site and more frequent acquisition by females of these sites as breeding sites the following year suggested a greater investment by adult females in female young. Therefore, competition between female siblings for the acquisition of their natal site may be sufficient to result in the greater production of males. In addition, the simultaneous occupation of, and competition between, siblings and parents for the resources of the natal home range may not be necessary for local resource competition to result in a greater production of the dispersing sex. Greater-than-expected binomial variance in sex ratios of litters suggested that adjustment of sex-ratios occurred. However, we were unable to associate the adjustment of litter sex ratios with changes in maternal condition. The greater production of males and the predominance of monogamous associations between adults in this population may have lead to slightly greater variation in male fitness than female fitness. Therefore, a female in better-than-average condition may have benefited by producing more males. Similarly, a lower cost of producing dispersing males may allow nutritionally-stressed females to reduce their total expenditure on offspring by producing more males. Because these experiments were non-manipulative, maternal condition may not have varied sufficiently during this study to detect adjustments of litter sex ratios resulting from either of the above mechanisms acting separately, but the combined effects of small differences in matermal condition and selective pressures operating in the same direction may have resulted in the observed deviation from the binomial.

Variable sex ratio among colonies of ants

Small colonies of ants often produce mostly male alates, while large colonies produce mostly female alates. I present a simple model consistent with this pattern in which males that compete for mates are related (Local Mate Competition). The model explains the observed trend even when relatedness among competing males is low, so that there is only a negligible effect on the predicted sex allocation ratio in the population. The reverse trend is expected when there is competition among related females for a limited resource, such as nest sites (Local Resource Competition); small broods are predicted to be mostly female and large broods are predicted to be mostly male.

Sex-Ratio Adjustment in Odocoileus: Does Local Resource Competition Play a Role?

Sex-Ratio Adjustment in Odocoileus: Does Local Resource Competition Play a Role?

TECHNICAL CHANGE, LOCATION, AND TRADE

ABSTRACT The difference between the slow adjustment of localized resources and the faster processes of technical change and product renewal causes a relocation pressure on existing production, Introduction of new products, techniques, and establishments is stimulated by opportunities to attain temporary monopoly profits and is restrained by sunk costs in older vintages and competition for local resources;. Introduction is more frequent in nodes with a rich and varied import structure. The process can he modeled as a chain of import, production, and export cycles with relocation. Conditions for introduction and change of location are analyzed by means of a spatial vintage model.

Hierarchical selection theory and sex ratios I. General solutions for structured populations

Models of sex-ratio evolution in structured populations are derived with G. R. Price's covariance form for the hierarchical analysis of natural selection (1970, Nature 227, 520–521). Previous work on competition among related males for mates (local mate competition), competition among related females for a limiting resource (local resource competition), inbreeding, group selection, and asymmetry of genetic inheritance between males and females, are subsumed under a general formulation for sex-ratio biases in structured populations. I found that the evolutionarily stable strategy sex ratio (males:females) for diploids is 1 − ϱ m :1 − ϱ f , where ϱ m is the regression coefficient of relatedness of the controlling genotypes on males competing for mates, ϱ f is the regression of controlling genotypes on females that compete for a fixed, limiting resource, and there is no inbreeding. For inbreeding and no competition among females, the evolutionarily stable strategy is 1 − ϱ m :1 + ϱ mf , where ϱ mf is the regression of controlling genotypes on females' mates.

Sex Ratios of Nestling and Fledgling Red-Cockaded Woodpeckers (Picoides borealis) Favor Males

Red-cockaded woodpeckers breeding in coastal South Carolina produce nestling and fledgling sex ratios dominated by males. Differences in the costs of daughters versus sons in terms of the growth of nestlings, an earlier death of sons, or local resource competition between parents and daughters are not evident. However, local resource enhancement is a possible source of the theoretically reduced costs of sons. Because red-cockaded woodpeckers breed in clans consisting of pairs alone and pairs assisted by adult male helpers, we hypothesized that progeny sex depends on the presence or absence of helpers. We found no strong support for this notion; nestling sex, however, depends on the tenure class of breeding females, suggesting that sex-ratio variation may be facultative (in the sense of Williams 1979) and related to kinship asymmetries among the clan types in this species.

The Adaptive Value of Male-Biased Sex Ratios Among Stressed Animals

The Adaptive Value of Male-Biased Sex Ratios Among Stressed Animals

Dominance Rank, Secondary Sex Ratio, and Reproduction of Offspring in Polygynous Primates

One question in evolutionary biology which is central to understanding reproductive strategies and parental efforts at rearing offspring is why organisms produce certain offspring sex ratios (Trivers and Willard 1973; Charnov 1982). Fisher (1930) argued that since each individual has a mother and a father, both sexes contribute genes equally to the next generation. All else being equal, if members of one sex were overproduced, their ability to reproduce would be limited by the availability of members of the less abundant sex and natural selection would favor any predisposition to produce offspring of the less abundant sex. In this fashion, Fisher reasoned, selection would adjust the offspring sex ratio in an outbreeding population until total parental investment (PI, Trivers 1972) in the two sexes was equal. Fisher (1930) assumed, but did not clearly state, that all parents were alike in their ability to rear reproductively successful sons and daughters (Patterson and Emlen 1980). A model proposed by Trivers and Willard (1973), however, specified how deviations from equal PI may be favored by natural selection according to the influence of maternal condition on the reproduction of offspring. The TriversWillard model proposed that in polygynous species if some condition of mothers (e.g., dominance rank) influences the condition and, as a result, the reproductive success (RS) of sons more than daughters, then selection should result in the evolution and maintenance of mechanisms by which females in poor condition invest less in sons than daughters and/or females in good condition invest more in sons than daughters. This model is particularly relevant for primates, since the majority of species are polygynous (Clutton-Brock and Harvey 1977). Since the dominance rank of a monkey's social group and family has a strong

Local resource competition and the evolution of male-biased sex ratios

In many mammalian species, maturing males disperse from their natal groups, while females remain near their places of birth. If sources are limited locally, related females may be forced to compete with one another for access to resources, while mature males will compete primarily with unrelated individuals in non-natal groups. Clark (1978) suggested that under such circumstances the extent of competition among females could be reduced if females uniformly skewed the sex ratio of their offspring in favor of males. Skewed secondary sex ratios and investment patterns in galagos and red deer appear to be consistent with this hypothesis. Several authors have recently demonstrated that the evolution of skewed sex ratios may be influenced by population structure, and Wilson & Colwell (1981) have shown that male-biased sex ratios can evolve in structured haplodiploid populations. Here, I construct an intrademic group selection model to simulate the effects of density-dependent mortality upon diploid populations structured much as Clark describes: males disperse, females remain near their female relatives, and local resource competition limits the size of local groups. The results confirm that male-biased sex ratios can evolve in a diploid species, even when groups are large and the skew in sex ratio is substantial.

Local Resource Competition and Facultative Adjustment of Sex Ratios in Relation to Competitive Abilities

I have suggested that local resource competition and male dispersal may generally lead to (1) intense competition among females for resources and reproductive opportunities, (2) limiting the recruitment of females, (3) extensive maternal investment in vulnerable female offspring, and (4) facultative adjustment of the sex ratio of progeny in relation to maternal competitive abilities. This argument appears to be consistent with evidence drawn from several species of macaques and baboons. The size of groups appears to be limited by the availability of resources; females actively compete over access to resources, and limit the number of females born and raised in their groups through the effective harassment of females carrying female fetuses and immature females. In response to such harassment, females appear to invest more in their daughters than sons, and to adjust the sex ratio of their progeny in relation to the expected reproductive success of their sons and daughters. High ranking females whose female progeny are likely to survive to become high ranking, reproductively successful adults produce more daughters than sons; while low ranking females whose female offspring are unlikely to survive to maturity or to reproduce successfully, produce more sons than daughters.